3. Nuclear Magnetic Resonance
It is the study of absorption of radiofrequency
radiation by nuclei in a magnetic field is called
Nuclear Magnetic Resonance.
Nuclear magnetic resonance spectroscopy is basically
another form of absorption spectrometry. It involve
change of the spin state of a nucleus, when the nucleus
absorb electromagnetic radiation in a strong magnetic
field.
4. The source of energy in NMR is radio waves
which have long wavelengths, and thus low energy
and frequency.
When low-energy radio waves interact with a
molecule, they can change the nuclear spins of
some elements having spin state 1/2, including 1H
and 13C.
5. A spectroscopic technique that gives us information
about the number and types of atoms in a molecule.
Nuclear magnetic resonance spectroscopy is a powerful
analytical technique used to characterize organic
molecules by identifying carbon-hydrogen frameworks
within molecules.
6. It is concerned with the magnetic properties of certain atomic
nuclei.
Involves change in the spin state at the nuclear level.
SPINNIG NUCLEUS
Proton acts as a tiny spinning bar magnet
and possesses both electrical charge and mechanical spin.
NMR is the most powerful tool available for organic structure
determination.
It is used to study a wide variety of nuclei:
1. 1H
2. 13C
3. 15N
4. 19F
5. 31P
7. Types of NMR
Two common types of NMR spectroscopy are used
to characterize organic structure:
1H NMR:- Used to determine the type and number
of H atoms in a molecule.
13C NMR:- Used to determine the type of carbon
atoms in the molecule.
8. Principle of NMR
The sample is dissolved in a solvent, usually CDCl3
(deutero-chloroform), and placed in a magnetic field.
A radio frequency generator then irradiates the
sample with a short pulse of radiation, causing
resonance.
When the nuclei fall back to their lower energy state,
the detector measures the energy released and a
spectrum is recorded.
9. Proton in different environments absorb at slightly
different frequencies, so they are distinguishable by
NMR.
The frequency at which a particular proton absorbs is
determined by its electronic environment.
The size of the magnetic field generated by the
electrons around a proton determines where it absorbs.
11. NMR Instrumentation
1. Sample holder
2. Permanent magnet
3. Magnetic coils
4. Sweep generator
5. Radio frequency transmitter
6. Radio frequency receiver
7. Read out systems
12. 1. Sample holder
Glass tube with 8.5 cm long, 0.3 cm in diameter.
2. Permanent magnet
It provides homogeneous magnetic field at 60 - 100 MHz.
3. Magnetic coils
These coils induce magnetic field when current flows through
them.
4. Sweep generator
To produce the equal amount of magnetic field pass through
the sample.
13. 5. Radio frequency transmitter
A radio transmitter coil that produces a short powerful pulse
of radio waves.
6. Radio frequency
A radio receiver coil that detects.
7. Read out system
A computer that analyses and record the data.
Radio frequencies emitted as nuclei relax to a lower
energy level.
Receiver
14. Induced Magnetic Field
When a molecule is placed in a magnetic field, its electrons
are caused to circulate and thus they produce secondary Magnetic
field is known as Induced magnetic field.
Shielding
If the Induced field oppose the applied field then the proton
will come to resonance only at a higher applied magnetic field (up
field). In these case proton is said to be shielded.
15.
16. Deshielding
If the induced magnetic field reinforce the applied field,
then the protons will come to resonance at a lower applied field
(down field). In this case the proton is said to be deshielded.
Ring current in benzene
17.
18. Chemical shift
Shifts in the position of NMR absorptions arise from the
Shielding and deshielding by electrons, due to different chemical
environments around protons are called chemical shift (δ-value or τ-
tau).
20. TMS (Tetra methyl silane) is most commonly used as a NMR
spectroscopy. Due to following reasons;
It is chemically inert and miscible with a large range of
solvents.
Its twelve protons are all magnetically equivalent.
Its protons are highly shielded and gives a strong peak even
small quantity.
It is less electronegative than carbon.
It is highly volatile and can be easily removed to get back
sample.
TMS Characteristics
21. The alternative system which is generally used for defining
the position of resonance relative to the reference is assigned
tau (τ) scale.
τ = 10 – δ
A small numerically value of δ indicates a small download
shift while large value indicates a large download shift.
A small value of τ represents a low field absorption and a
high value indicates a high field absorption.
τ - scale
22. Factors affecting chemical shift
(a) Inductive effect
(b) Vanderwaals deshielding
(c) Anisotropic effect
(d) Hydroden bonding
a). Inductive effect or electronegative groups
Electronegative atoms (or) groups attached to the C-H system
decrease the electron density around the protons, and there is less shielding
(i.e. deshielding) and chemical shift increases.
Compound Chemical shift (δ)
TMS 0
CH4 0.4
CH3I 2.16
CH3Br 2.68
CH3Cl 3.05
CH3F 4.26
23. Similarly, increase in the substitution of an electronegative
atom for hydrogen δ value increase.
Compound Chemical shift (δ)
CH3Cl 3.05
CH2Cl2 5.3
CHCl3 7.3
b). Vanderwaal’s deshielding
In overcrowd molecules that some proton occupying sterically
hindered position. Clearly electron cloud surrounding the proton.
Therefore the proton is shielded and δ value increases.
24. c). Anisotropic effect (Space effect)
Due to the presence of π-bond
Anisotropic effects constitute shielding and deshielding effects
on the proton because of induced magnetic fields in other parts
of the molecule which operate through space.
For example, if a magnetic field is applied to a molecule having
π electrons, these electrons begin to circulate at right angles to
the direction of the applied field thereby producing induced
magnetic field.
25. If a magnetic field is applied to a molecule having π electrons, these
electrons begin to circulate at right angles to the direction of the applied
field thereby producing induced magnetic field.
The effect of this field on the nearby proton has been found to depend
upon the orientation of the proton with respect to the π bond producing
the induced field.
Ring current in benzene
26.
27.
28. d). Hydrogen bonding
The hydrogen bonded proton being attached to a highly
electronegative atom will have smaller electron density around it.
Being less shielded, the field felt by such a proton will be more and
hence resonance will occur downfield.
The downfield depends upon the strength of hydrogen bonding.
Then used to differentiate inter and intra molecular hydrogen
bonding.
Inter molecular hydrogen bond δ value are temperature
and concentration dependent.
Intra molecular hydrogen bond δ value are temperature
and concentration independent.
30. Proton NMR (1H NMR)
The most common used for NMR is based on
the hydrogen-1 (1H), nucleus or proton.
It can give information about the structure of
any molecule containing hydrogen atoms.
33. 1H NMR spectrum of Ethanol
1H NMR spectrum of Ethanol;
3 types of protons (CH3, CH2, OH)
34. Aromatic Hydrogen shows peak in the chemical shift scale
6.5–8.0 ppm.
From the above spectrum benzene has same type of protons and
it shows single peak at 7.26.
1H NMR spectrum of Benzene
35. Features of 1H NMR
Natural abundance of 1H is 99.9844.
PNMR is to determine type and number of H-
protons in a molecule.
The source of energy in NMR is radio waves
which have long wavelengths, and thus low energy
and frequency .
The chemical shift range of PNMR is 0 to 14 ppm.
36. PNMR is having coupling constant range 0 to 15 Hz.
The solvent used for dissolving sample should have
following properties;
Should not contain proton,
Inexpensive
Low boiling point and non polar in nature.
Generally deuterated chloroform CDCl3 is used as
solvent.
37. TMS is used as internal standard.
Sodium salt of 3-(trimethyl silyl) propane sulphonate
is also used as solvent, which is a water soluble
solvent.
In PNMR ,continuous wave method is used.
NMR absorptions appear as sharp peaks.
There are three types of Proton isotopes used in
NMR,1Hydrogen, 2Deuterium, 3Tritium.
38. Interpretation of 1H-NMR spectra
Number of signals - Indicates how many “ different
kinds of protons are present.
Position of signals - Indicates something about (chemical shift)
magnetic (electronic environment of
protons.
Relative intensity - Proportional to number of protons present.
of signals
Splitting of signals - Indicates the number of nearby nuclei
(spin spin coupling) usually protons
39. Number of Signals
The number of signals in the NMR spectrum tell
the number of different set of equivalent proton in
a molecule. Each signal correspond to the set of
equivalent proton.
It may be noted that magnetically equivalent
proton are chemically equivalent proton.
48. The following solvents are normally used in which
Hydrogen replaced by deuterium.
CCl4 – carbon tetrachloride
CS2 – carbon disulfide
CDCl3 – Deuteriochloroform
C6D6 – Hexa deuteriobenzene
D2O – Deuterium oxide
Solvents used in NMR
49. n+1 rule
The multiplicity of signal is calculated by using n+1 rule.
This isn one of the rule to predict the splitting of proton
signals. This is considered by the nearby hydrogen nuclei.
Therefore, n= Number of protons in nearby nuclei
Zero H atom as neighbour n + 1 = 0 + 1= 1 (singlet)
One H atom as neighbour n + 1 = 1 + 1= 1 (doublet)
Two H atom as neighbour n + 1 = 2 + 1= 3 (triplet)
50. 1
1
1
1
1
1 1
1 1
1
1
1
1
2
3 3
5
4
5
46
10 10
6 15 20 15 6
singlet
doublet
quintet
quartet
triplet
sextet
septet
PASCAL’S TRIANGLE
Intensities of
multiplet peaks
The interior entries are the
sums of the two numbers
immediately above.
51. Spin-spin coupling (splitting)
The interaction between the spins of neighbouring
nuclei in a molecule may cause the splitting of NMR
spectrum. This is known as spin-spin coupling or
splitting.
The splitting pattern is related to the number of
equivalent H-atom at the nearby nuclei.
Example : Ethyl acetate
53. Rules for spin-spin coupling
Hb couple with Hc
Hb and Ha do not couple because they are equivalent.
Hc and Hd do not couple because they are equivalent.
Ha can couple with Hb
Ha can couple with Hc
Ha cannot couple with Hd
Chemically equivalent protons do not show spin-spin coupling.
Only nonequivalent protons couple.
Protons on adjacent carbons normally will couple.
Protons separated by four or more bonds will not couple.
54. Coupling constant (J)
The distance between the peaks in a given multiplet is a
measure of the splitting effect known as coupling constant.
It is denoted by symbol J, expressed in Hz.
Coupling constants are a measure of the effectiveness of
spin-spin coupling and very useful in 1H NMR of complex
structures.
56. 13C-NMR
Proton NMR used often for the complete elucidation of the
unknown compound.
Carbon NMR can used to determine the number of non-
equivalent carbons and to identify the types of carbon atoms
(methyl, methylene, aromatic, carbonyl….) which may present
in compound.
12C has no magnetic spin. 13C has a magnetic spin, but is only
1% of the carbon in a sample.
The gyromagnetic ratio of 13C is one-fourth of that of 1H.
Signals are weak, getting lost in noise. Hundreds of spectra are
taken, averaged.
57. Characteristic features of 13C NMR
The chemical shift of the CMR is wider (δ is 0-240ppm
relative to TMS) in comparison to PMR (δ is 0-14ppm
relative to TMS).
13C-13C coupling is negligible because of low natural
abundance of 13C in the compound. Thus in one type of
CMR.
Spectrum (proton decoupled) each magnetically non
equivalent carbon gives a single sharp peak that does
undergo further splitting.
58. The area under the peak in CMR spectrum is not
necessary to be proportional to the number of carbon
responsible for the signal. Therefore not necessary to
consider the area ratio.
In proton-coupled spectra, the signal for each carbon
or a group of magnetically equivalent carbon is split
by proton bonded directly to that carbon and the n+1
rule is followed.
59. Types of 13C spectra
1) Proton coupled 13C spectra
2) Proton decoupled 13C spectra
a) Homoannular coupling
1) Proton coupled 13C spectra
The probability of finding 13C adjacent carbon is very
less. Therefore, homonuclear [carbon-carbon] splitting is
rearaly seen.
b) Hetronuclear coupling
It involving two different atoms [carbon- hydrogen].
Here splitting arises due proton attached directly to 13C carbon.
60.
61. Here the decoupling technique obliterates all the
interaction between proton and 13C nuclei thus singlet
are observed in proton decoupled 13C spectra.
2) Proton decoupled 13C spectra
62.
63.
64. Interpreting 13C NMR
The number of different signals indicates the number
of different kinds of carbon.
The location (chemical shift) indicates the type of
functional group.
The peak area indicates the numbers of carbons (if
integrated).
The splitting pattern of off-resonance decoupled
spectrum indicates the number of protons attached to
the carbon.
65.
66.
67. General Applications of NMR Spectroscopy
NMR is used in biology to study the biofluids, cells, per fused
organs and biomacromolecules such as nucleic acids (DNA,
RNA), carbohydrates, proteins and peptides. And also
Labeling studies in biochemistry.
NMR is used in physics and physical chemistry to study high
pressure diffusion, liquid crystals, liquid crystal solution,
membranes, rigid solids.
NMR is used in food science.
68. NMR is used in pharmaceutical science to study Pharmaceuticals
and drug metabolism.
NMR is used in chemistry to;
Determine the enantiomeric purity
Elucidate chemical structure of organic and inorganic
compounds.
Macromolecules – ligand interaction.
69. Anatomical imaging.
Measuring physiological functions.
Flow measurements and angiography.
Tissue perfusion studies.
Tumors.
Applications of NMR in medicine
MRI is specialist application of multi dimensional Fourier
transformation NMR.
